Poster at the 40th annual meeting of the Biopysical Society . Abstract: Biophys. J. 70, A427 Cultured skeletal muscle cells from fetal rats exhibit spontaneous rhythmic fluctuations in intracellular calcium activity after approximately 5-6 days in culture. We have observed two types of spontaneous calcium activities: (1) waves of calcium which appear to originate from a single point source and (2) calcium sparks that originate in a repetitive stochastic manner from multiple sites; the majority of which are localized to one region of the cell. Calcium waves propagate throughout the myotube, even in the absence of the characteristic ~twitching ~ normally exhibited by these cultured cells, although waves are always found in actively twitching cells. The sparks remain localized to the region of origin and occasionally can be seen as rings of calcium, a few microns in diameter, presumably released from internal structures. Using two photon excitation point scanning (6 frames/sec) and line scanning (30 frames/sec) microscopy we have measured the rate of propagation and the frequency of both the waves and sparks in Fluo-3 and Indo-1 loaded cells and suggest possible sources of these two phenomenon.